The Intricate Web of Asymmetric Processing of Social Stimuli in Humans

Abstract

Although the population-level preference for the use of the right hand is the clearest example of behavioral lateralization, it represents only the best-known instance of a variety of functional asymmetries observable in humans. What is interesting is that many of such asymmetries emerge during the processing of social stimuli, as often occurs in the case of human bodies, faces and voices. In the present paper, after reviewing previous literature about human functional asymmetries for social and emotional stimuli, we suggest some possible links among them and stress the necessity of a comprehensive account (in both ontogenetic and phylogenetic terms) for these not yet fully explained phenomena. In particular, we propose that the advantages of lateralization for emotion processing should be considered in light of previous suggestions that (i) functional hemispheric specialization enhances cognitive capacity and efficiency, and (ii) the alignment (at the population level) of the direction of behavioral asymmetries emerges, under social pressures, as an evolutionary stable strategy.

Broadening the scope: Increasing phenotype diversity in laterality research

“…We suggest that incorporating social laterality phenotypes (Marzoli et al, 2022 ) in studies on the ontogenesis and evolution of hemispheric asymmetries would benefit research in several ways. Laterality in social interactions has been found across many behavioral dimensions (Ocklenburg et al, 2018 ).…”

Environmental and genetic determinants of sensorimotor asymmetries in mother-infant interaction

“…Either point seems to be consistent with both phylogenetic and ontogenetic explanations. For instance, the significantly larger exposure to right-handed rather than left-handed individuals could entail-at either ontogenetic or phylogenetic levels, or both (e.g., see Marzoli et al, 2014Marzoli et al, , 2022Lucafò et al, 2021 for similar considerations)a perceptual and attentional bias toward the right side of others' body, broadly corresponding to a bias for the left visual field (from an allocentric perspective), as well as a leftward bias/right-hemispheric advantage for the processing of emotions from faces, which in turn could foster an overall dominance…”

“…The potential advantages of lateralization for the processing of emotions are in line with previous theoretical proposals (e.g., Vallortigara and Rogers, 2005 , 2020 ) that functional hemispheric asymmetries might enhance cognitive efficiency (e.g., a positive association has been found between the typical leftward bias for emotional faces and better performance in emotion recognition; Workman et al, 2006 ; Watling and Bourne, 2013 ), and that the directional alignment of behavioral biases (at the population level) might respond to an evolutionary stable strategy molded by social pressures (e.g., the leftward bias for emotional faces would favor the monitoring of the dominant hand of others and their emotional states within the same hemisphere/perceptual field; Marzoli et al, 2014 , 2022 ). Either point seems to be consistent with both phylogenetic and ontogenetic explanations.…”

“…Either point seems to be consistent with both phylogenetic and ontogenetic explanations. For instance, the significantly larger exposure to right-handed rather than left-handed individuals could entail—at either ontogenetic or phylogenetic levels, or both (e.g., see Marzoli et al, 2014 , 2022 ; Lucafò et al, 2021 for similar considerations)—a perceptual and attentional bias toward the right side of others' body, broadly corresponding to a bias for the left visual field (from an allocentric perspective), as well as a leftward bias/right-hemispheric advantage for the processing of emotions from faces, which in turn could foster an overall dominance of the right hemisphere for emotions. Moreover, according to the phylogenetic view, the LCB might have evolved from the right-hemisphere dominance for emotion processing (e.g., see Palomero-Gallagher and Amunts, 2022 ) in the visual, auditory or tactile domains (Harris et al, 2001 , 2010 ; Sieratzki and Woll, 2002 ; Bourne and Todd, 2004 ; Vauclair and Donnot, 2005 ; Donnot, 2007 ; Huggenberger et al, 2009 ).…”

“…Although these studies seem to suggest a partially common ground among genetic variants affecting the development of brain functional lateralization and the occurrence of neurodevelopmental disorders (Wiberg et al, 2019 ), at present no single specific gene has yet been identified for either perceptual or motor asymmetries (e.g., the statistical frequencies of hand preference observed in families), which probably have a polygenic basis (Medland et al, 2009 ; McManus et al, 2013 ; Armour et al, 2014 ). In our opinion, the effects of epigenetic factors acting on the basis of genetically driven “core asymmetries” and of environmental influences should be taken into account when attempting to explain the origin of asymmetries in the processing of social stimuli and related behaviors (Marzoli et al, 2022 ). In this regard, it has not yet been studied which genetic and epigenetic factors affect the direction of cradling lateralization, nor which epigenetic changes can be induced by typical or atypical maternal cradling lateralization on the offspring.…”